Seunghun Baek

Design and development of Generation-I silicon based Solid State Transformer

The Solid State Transformer (SST) is one of the key elements proposed in the National Science Foundation (NSF) Generation-III Engineering Research Center (ERC) “Future Renewable Electric Energy Delivery and Management” (FREEDM) Systems Center. The SST is used to enable active management of distributed renewable energy resources, energy storage devices and loads. In this paper, the Generation-I SST single-phase 20kVA, based on 6.5kV Si-IGBT is proposed for interface with 12kV distribution system voltage. The SST system design parameters, overall system efficiency, high frequency transformer design, dual active bridge converter, auxiliary power supply and gate drives are investigated. Design considerations and experimental results of the prototype SST are reported.

Design considerations of high voltage and high frequency three phase transformer for Solid State Transformer application

The three-phase Solid State Transformer (SST) is one of the key elements in the Future Renewable Electric Energy Delivery and Management (FREEDM) System. The SST consists of an input rectifier, Dual Active Bridge (DAB) bidirectional dc-dc converter and followed by an inverter for ac voltage output. The DAB converter is a bidirectional dc-dc converter using high frequency transformers to step up or down the voltages at high frequency for a reduction in size while maintaining high efficiency and reliability. The single phase high-frequency high-voltage transformer for two-level DAB has been reported by the authors [2]. In this paper, high frequency three-phase transformer for multi-level (3-level) and multi-phase (three-phase) DAB is designed for three-phase multilevel SST. There are two proposed topologies and for each topology the high voltage and high frequency transformer is designed at 3 kHz and 20 kHz. The proposed transformer designs for electromagnetic, thermal and mechanical analysis are validated with the simulation of Finite Element Analysis software.

Design and hardware implementation of Gen-1 silicon based solid state transformer

This paper presents the design and hardware implementation and testing of 20kVA Gen-1 silicon based solid state transformer (SST), the high input voltage and high voltage isolation requirement are two major concerns for the SST design. So a 6.5kV 25A dual IGBT module has been customized packaged specially for this high voltage low current application, and an optically coupled high voltage sensor and IGBT gate driver has been designed in order to fulfill the high voltage isolation requirement. This paper also discusses the auxiliary power supply structure and thermal management for the SST power stage.

High-voltage high-frequency transformer design for a 7.2kV to 120V/240V 20kVA solid state transformer

Solid state transformer (SST) exhibits good features such as high power density, small volume and weight, controlled power factor, voltage sag ride through, etc. compared with traditional line frequency transformer. The 7.2kV AC to 120V/240V AC 20kVA solid state transformer is a key component of the future renewable electric energy delivery and management (FREEDM) systems as the interface between the 7.2kV distribution grid and the low voltage residential micro-grid. Three cascaded 6.7kVA high-voltage high-frequency transformers operating at 3kHz are employed to convert voltage from 3800V high voltage DC link of each cascaded stage to 400V low voltage DC link. The transformer is required to withstand at least 15kV high frequency voltage insulation continuously. Transformer magnetic core materials were reviewed and compared. Winding layout alternatives for leakage, magnetizing inductance and insulation were compared. An insulation strategy based on split core and separate winding structure with inserted insulation layer between the C cores was proposed. One 6.7kVA high voltage high frequency transformer prototype was built and the test results were reported.

High-frequency design considerations of dual active bridge 1200 V SiC MOSFET DC-DC converter

Silicon carbide (SiC) is more favorable than Silicon (Si) to build high voltage devices due its wider band-gap and higher critical field strength. Especially, the SiC MOSFETs are finding their niche in 1 kV range, which is currently dominated by Si IGBTs. This paper aims at demonstrating high power and high frequency operation of the SiC MOSFETs, as a means to evaluate the feasibility of using SiC MOSFETs for high power density applications. The sample devices chosen for this study are 1200 V, 20 A, SiC MOSFETs co-packed with 10 A JBS diodes — manufactured by the CREE Inc. A dual active bridge (DAB) converter has been built to validate the suitability of SiC devices for high power density converters. The design details of the DAB hardware, and the high frequency transformer used for interfacing both the bridges are given. Experimental results on the DAB at 100 kHz switching frequency are presented. Finally, the device switching waveforms up to 1 MHz are given.

Improved wind farm’s power availability by battery energy storage systems: modeling and control

This paper presents the study on the integration of battery energy storage system (BESS) to STATCOM for smoothing out the intermittent output power of a wind farm. Lead-acid battery type is considered as the energy storage system for availability and comparably simpler management system. Reviewed several modeling approaches indicating their advantages and drawbacks; particularly, appropriate for high power applicationspsila time-frames, current-based third order model is developed. The experimental tests are conducted to identify the parameters and consequently, to construct the battery model. The performance of the proposed system is analyzed by introducing the state-of-charge (SOC) controller implemented for this type of battery integrated to a proposed 10 MVA STATCOM for a 50 MW wind farm.

Characterization of a three-phase dual active bridge DC/DC converter in wye-delta connection for a high frequency and high power applications

In this paper, high-frequency (HF) three-phase dual active bridge (DAB) dc/dc converter in wye-delta connection is characterized and the optimal condition in efficiency and size of the practical application is investigated. The proposed topology has many advantages, such as linear operation and wide range of full ZVS operation region which have been the major restrictions of the existing single-phase DAB applications. This new approach is applied to a 10kW DAB dc/dc converter prototype with multiple outputs to mitigate the switching losses using newly introduced 1200V SiC MOSFET and 1200V JBS Diodes.

Power flow analysis for 3-port 3-phase dual active bridge dc/dc converter and design validation using high frequency planar transformer

In this paper, an inductor-integrated three-winding shell-type planar transformer is designed and analyzed for three-port dual-active bridge (DAB) dc/dc converters in wye wye-delta connection. The steady-state operation principle of the proposed topology has been studied and design parameters of the isolation transformer are analytically determined. The proposed geometry and design method for the ac-link transformer allows us to integrate a large number of bulky inductors required for three phase DAB operation to the transformer without additional connections and unanticipated parasitic effects. It also considerably simplifies the equivalent leakage inductance circuit model and power flow analysis. This configuration is suitable for high power and high step up/down ratio dc/dc converter applications which requires series and/or parallel combination of converters. The experimental and FEM simulation results from prototypes are presented and it validates the theoretical considerations and feasibility of the proposed approach for isolated dc/dc converter applications such as solid-state transformer (SST).

Analytical modeling of a medium-voltage and high-frequency resonant coaxial-type power transformer for a solid state transformer application

This paper introduces a unique medium-voltage and high-frequency resonant coaxial-type power transformer (RCT) for a power-distribution level Solid State Transformer (SST) application [1]. The dc-dc stage of the SST application requires a compact high performance resonant transformer that operates under high electric stress and frequency. The RCT discussed in this work brings the virtues of the coaxial-type transformer, which has been use in radio frequency applications, to medium-voltage power conversion applications. The high frequency parasitic effects, which are negligible at 60Hz, become a significant concern in a SST operating above kHz range. Hence, the parasitics of the transformer need to be accurately predicted and controlled for quality power control and safety. The RCT not only minimizes the parasitics of the transformer but also integrates a resonant tank comprised of a series inductance and shunt capacitance by utilizing the stray magnetic and electric flux within the transformer. The limited space in compact size is efficiently used and materials can be optimized. The unique analytic design method of an RCT is introduced and the equivalent circuit model is developed in this paper. The specific design is based on the high performance transformer for a dual-active bridge (DAB) converter, which is one of the most popular topologies in bidirectional power conversion, in the dc-dc stage of the SST application being developed at the FREEDM System Center and its concept is verified with FEM analysis and experiments.

Accurate equivalent circuit modeling of a medium-voltage and high-frequency coaxial winding DC-link transformer for solid state transformer applications

The 12kV-400V dc-dc stage of a distribution level solid state transformers (SST) has been under research and development. Development of a 15kV SiC Mosfet allows a single stage of the dc-dc converter to operate at medium voltage at an operating frequency of over 20kHz. Nonetheless, the high rising and falling time during pulse switching in the dual active bridge operation is another significant obstacle to realize this technology. In order to understand and predict the frequency response with pulse switching and consider common-mode response via circuit analysis accurately, lumped-element equivalent circuit model has been developed for broadband coaxial winding transformer (CWT) with analytic expressions. The simple lumped-element equivalent circuit introduced in this paper has been verified by measurement results from a prototype for a medium-voltage (MV) and high frequency (HF) coaxial winding power transformer up to the frequency where the length of the coaxial body is a quarter of a wavelength and further study up to 30MHz has been described.